1. Field of the Invention
The present invention relates to the field of radio frequency identification (RFID), and, more particularly, to a radio frequency identification reader and to a method for locating a tag using the radio frequency identification reader.
2. Description of the Related Art
Compared with high frequency (HF) radio frequency identification (RFID) systems, ultra high frequency (UHF) RFID has many superior properties, such as fast tag accessing speed, cheap tags, and the ability to perform tagging at the level of individual items in both manufacturing and logistics applications. However, in view of the far-field electromagnetic transmission characteristics of UHF RFID, for instance, multi-path transmission, it is difficult to maintain a controlled reading zone for UHF RFID. There may be certain reading gaps (i.e., field nulls) in the required reading zone, which make reading less reliable in that zone. Cross reading may also occur during identification of remote tags outside the zone. This lack of control over the reading zone causes major problems in manufacturing and logistics applications.
In manufacturing applications, the system must ascertain precisely when a tag enters the reading zone, and when the tag leaves the reading zone. Taking workpiece identification applications as an example, the system must ascertain precisely which tags are really inside the reading zone, in order to subject the workpieces attached to these tags to further processing. However, multi-path transmission may cause the system to mistakenly identify tags outside the reading zone, and correspondingly to execute an erroneous processing command on the workpiece which is about to arrive.
There may be similar problems in logistics applications:
(1) In luggage tracking applications at airports, the lack of control over the reading zone results in difficulty in distinguishing between tagged luggage bags when they pass the antenna. To solve this problem, an expensive box made from material that absorbs radio waves must be used to control the reading zone.(2) In forklift applications with multiple inlets, signal leakage leads to the reader cross-reading tags of inlets that are not the object of interest. As a result, it is very difficult to correctly identify movement of goods through different inlets.(3) There is a similar impact in applications where the load of a goods forklift is to be accurately identified. The multi-path problem results in a forklift equipped with an RFID system being unable to effectively distinguish between a background tag and a tag on the forklift pallet.
It is evident that all of the application scenarios mentioned above require a controlled reading zone: within the required reading zone, tags can be read reliably and there are no field nulls; outside the reading zone, cross reading will not occur. It must be pointed out that control over reading zones in UHF RFID is an excellent research subject both from an industrial and an academic perspective. Existing solutions can be divided into two types: the first provides a controlled reading zone by improving and utilizing current RFID communication methods, while the second uses auxiliary measures to enhance current RFID systems.
I. The first type of solution concentrates on the following two aspects:
1. Using a Far-Field Antenna to Filter Remote Tags, and Thereby Controlling the Reading Zone
(1) OMRON proposes some solutions in US20070241904A1 and US20080198903A1, according to which the tag distance is measured by detecting phase difference in reflected carriers so as to perform distance filtering.
To overcome the multi-path problem, US20070241904A1 uses different frequencies to communicate with tags at different times, and records the phase shift of backscattered carriers on each frequency, so as to measure tag distance on each frequency. However, this solution is unable to detect a phase difference of 360 degrees when a tag moves a distance of half a wavelength.
US20080198903A1 discloses a solution for locating a tag based on phase differences in two carriers of different frequencies. Ambiguity of distance measurement will not occur in this solution, but this solution can only analyze static phase differences on the two frequencies, and so can only obtain one fixed distance of a tag. Moreover, this solution is likely to be affected by the surrounding environment; for example, the phase differences will be affected by all reflective objects. If there is a metal plate very close to the tag, it will be impossible to distinguish between a reflection from the tag and a reflection from the metal plate by analyzing the phase differences. To solve this problem, the pattern in which phase difference varies with time must be analyzed. Because there is an ambiguity of 360 degrees in a conventional IQ structure on each frequency, however, this solution is unable to obtain the above pattern.
(2) Intermec proposes a solution in which phase change in a conventional IQ structure is detected when a single carrier is used, and also proposes three phase difference of arrival (PDOA) methods in the time domain, frequency domain and spatial domain.
(3) Pavel et al. propose a solution capable of accurately locating a tag based on the phase difference between two or more receiving antennas.
Since only a single carrier is used each time in solutions (2) and (3), these solutions have a similar flaw to that of solutions (i) (proposed by OMRON).
(4) In CN0160421A, AutoID Fudan proposes a solution based on direct sequence spread spectrum (DSSS) technology. In this solution, the backscattered signal from the tag is spread using DSSS technology. The reader then subjects the signal reflected from the tag to distance measurement based on the time difference (time of flight (TOF), using fast correlation. It must be pointed out that the reliability and accuracy of this solution are dependent on the speed of the pseudorandom noise (PN) code to a large degree. However, owing to the spreading operation, the use of a high-speed PN code will introduce a need for greater bandwidth, which cannot be supported by many existing RFID communication specifications.(5) Different solutions have been proposed by Alien and Impinj, in which the reading zone is controlled indirectly by measuring the direction and speed of movement of the tag. However, this solution is only effective in scenarios involving moving tags. A new solution is required for scenarios involving stationary tags, i.e., this solution is not suitable for use with both stationary tags and moving tags.2. Using a Near-Field (NF) Antenna to Control the Reading Zone
Another feasible solution to the problem of control over reading zones is a near-field UHF antenna. In this solution, a magnetic coupling scheme replaces radio wave transmission. Furthermore, the far-field gain of the antenna can be designed to be very small (e.g., −20 dBi). Thus it can be used to construct a controlled reading zone (see US20080048867A1). However, wavelengths are very small (about 30 cm) in the UHF band, and it is difficult to design an NF antenna with a large reading distance. Moreover, the far-field gain is proportional to the NF reading distance. Most NF antennas have a reading distance of about 5 cm; the maximum distance of a commercially available NF antenna is just 15 cm, and the far-field gain thereof can be as high as 6 dBi. Therefore, it is very difficult to design an NF antenna with a long NF distance and a small far-field gain.
II. The main feature of the second type of solution is the introduction of auxiliary measures, and the attempt to obtain a controlled reading zone using these auxiliary measures.
(1) A solution proposed by Fujitsu involves using an infrared sensor on an antenna support to detect when a tag enters or leaves the reading zone, and setting a time for antenna switching based on the above information, so as to improve the efficiency of reading/writing large amounts of data relating to multiple moving tags.(2) Different solutions have been proposed by Sverre Holm et al. and Mary Catherine et al. for integrating ultrasound into an RFID system. In these solutions, ultrasound may be used independently to obtain distance information for further processing, or a tag may be located based on the difference in ultrasound and RF transmission times.(3) Another conventional solution uses a material that absorbs radio waves to restrict transmission of radio waves. For instance, in airport luggage processing applications, a box made from an expensive material that absorbs radio waves is used to cover the required reading zone, and only tags that pass through the box will be read.
All these solutions require the use of new measures to obtain a controlled reading zone, increasing the cost of the entire system considerably. In some cases, the new measure (e.g., the box for absorbing radio waves) has a higher cost than the RFID system.